Drug side effects are a common cause of morbidity and mortality in the United States. It has been estimated that adverse reactions to drug therapy are the fourth to sixth leading cause of death in US hospitals. In addition, the financial burden of these events is substantial, with costs in excess of many billions of dollars per year. A certain proportion of these adverse reactions are due to drug-drug interactions, a process by which the administration of one drug alters the concentration of a second co-administered drug. The effect of one drug on the concentration of another can be due to the induction of drug metabolizing enzymes in the liver and gut, An improved understanding of this phenomenon at the molecular level is vital for addressing this problem. The overall goal of this research is to define structure-activity relationships regarding the promiscuous and differential activation of PXR isoforms that will be useful in rational drug design, thus predicting and avoiding lethal drug interactions. Determining the structural basis for the differential responsiveness of PXR.1 and PXR.2 to exogenous and endogenous compounds is vital in understanding the molecular mechanisms underlying the clearance of harmful chemicals from the body and will improve our ability to predict and avoid dangerous drug interactions. Clearly the two isoforms of PXR play critical roles in regulating xenobiotic and endobiotic homeostasis. The differential sensitivities of PXR.1 and PXR.2 suggest that each isoform plays a distinct role in regulating these processes. A more complete understanding of the structure of the PXR.1 and PXR.2 ligand-binding domains will certainly facilitate predictions about new drugs that are likely to interact with these sensors. Characterizing the ligand-binding pockets of both isoforms of PXR in the presence of structurally diverse ligands is the first step in developing the type of information useful in assessing the likelihood that new therapeutic candidates will affect the genes that regulate drug metabolism. Although the LBD of PXR has been crystallized, no such information is available for PXR.2. The first goal of this project is to express and purify the PXR.2 LBD in sufficient quantities to crystallize it. The second goal will be to repeat the crystallization of both LBDs in combination with a wide array of compounds and map the binding sites within the ligand-binding pockets of PXR.1 and PXR.2.